Author's note: This Instructable was written and published by our Making and Tinkering Programs Manager, who's personal Instructable account we co-opted. He is now publishing personal Instructables over at member ID MechaNickW.

We bought one of these glow-in-the-dark dog balls a number of years ago. When it was charged up and working from the sun it was great! But my main complaint was that it lasts just a few minutes, is best right after sunset when the ball has been sitting outside all day, and it recharges very slowly just sitting under a regular bulb inside. I like the concept (and so does our dog) but it needed something to make it more useful that we can play with over the winter when it gets dark around 5pm.

So, when the annual Make it Glow contest came around this year, I decided it was finally time to create an inventive solution to this problem. This Instructable details a relatively cheap, easy solution to quickly charge up the glowing dog ball for year-round use and hours of fun for your dog!

Glow-in-the-dark items are easily and quickly recharged using UV light (also called black light). Yes, one could very easily just buy a UV flashlight and shine it on the ball for a few seconds; however, this has a few drawbacks. For one, it's hard to find those flashlights in the dark and get the switch turned on. Also, my experience with the very cheap ones is that they are not very durable. Case in point, the light I'm using in this Instructable was one we used at my work which was broken, and something was wrong with the switch. Instead of throwing the thing away, I took it apart to salvage the UV LED assembly for this build.

When thinking about my design considerations for this project, I wanted it to be cheap (I literally purchased nothing specifically for this, just using things already on my workbench or from other projects) and I also wanted the trigger for turning on the charger to be pressure-sensitive. You literally just set the ball on the "charger", the UV light comes on for a few seconds, then you throw the ball for a bit til it stops glowing - my dog can do this all night, I swear! I also designed it with replication in mind for other users, please share if you decide to make one.

If you find this Instructable useful or inspiring, please vote for it in the Make it Glow contest.

Supplies

Glow-in-the-dark dog ball - this Instructable was designed around the very common Chuck-it Max Glow ball, available at most pet stores and online.
UV flashlight - this Esco UV flashlight is the one I took apart and hacked, but many of the ultra-cheap ones on Amazon are going to be the same or similar.
Battery pack - you can use either a 3-battery or 4-battery pack, and either AA 1.5 volt or AAA 1.5 volt batteries. The voltage tolerance of the LED array seems to be forgiving for between 4.5v-6v. I had this one from Adafruit kicking around already, so used it. You can also reuse the battery cylinder from the light if you solder some wires onto the terminals.
Batteries - 3 or 4 AA or AAA depending on your power source
3D-printed enclosure - see source files in a later step. I used glow-in-the-dark PLA filament but any works.
Microswitch - similar to the ones commonly used for 3D printer endstops.
Laser-cut clear acrylic disk, 1/8"(3mm) thick (if you can't make that happen, I will describe an alternative way to avoid it)
A soldering iron and solder
Wire strippers
Hot glue or 2-sided tape

Step 1: 3D Print the Enclosure and Laser-cut Acrylic LED Cover

To make a pressure-sensitive enclosure that switches on the light whenever you set the ball inside of it, you'll need to download and 3D print the attached enclosure. It took about 3.5 hours on my printer. I printed with supports just for the channel on the bottom to run wires up inside, but it actually should be fine without. 25-30% infill is sufficient, low-resolution layer height is fine too. Of course, I used glow-in-the-dark PLA, but whatever color you have is fine.

The rectangular socket for the microswitch should work for a variety of them - the hole for the UV LED assembly might need to be tweaked depending on yours, measure things up and modify before printing. If you are using the exact flashlight specified here, things should all fit pretty well if your printer is well-calibrated.

I had a laser-cut clear acrylic disc leftover from this project, so just used that. It's just one vector cut on a laser-cutter, no engraving, and the circle should be between 65-67mm in diameter. I'm not uploading a .dxf file because I don't have one and mocked this up in about 5 minutes - it is quick to do in most proprietary laser-cutter software interfaces, and the cut will take less than a minute. 1/8" (3mm) plastic is fine, if you have 1/4" (6mm) that will work as well, use what you have but it needs to be clear. If you have a glass lens about this diameter use that.

If you can't get access to a laser cutter, keep the lens from the flashlight and glue a small strip of plastic or wood to push down the button of switch, as shown later. It's not quite as useful, but will work in a pinch. Share in the comments if you can think of an alternative method, I like to use laser cutters and 3D printers but recognize not everyone has access to one or both!

Dog Ball Glow Charger AA.stl
Download View in 3D

Step 2: Take Apart the Flashlight

Okay, time to disassemble your suspiciously cheap UV flashlight. SAFETY WARNING: I take no responsibility if you shine this device or the flashlight in your eyes and burn out your retinas or something. It does emit UV light that is marginally harmful if you stare at it for long durations, we're removing the filter but only turning on the device when the light is obstructed by the ball. Use common sense.

The light I had was broken, so instead of throwing it away I decided to use it for this project. This means I got to spend a bit of time exploring how these work - they are relatively straightforward, and I would suspect a lot of other similar UV flashlights of this price point on the market are exactly the same or similar. Thread off the front of the light with the lens cover. Take out the UV LED assembly. Throw the rest away or use for something else creative.

There will be a spring on the back of the LED assembly - you'll want to desolder this from the light, just touch your solder iron to it for a few seconds and pull off. The spring is the positive (+) terminal for the assembly, which goes to the battery pack. There are some larger solder joints at the very edge of the assembly - these are all the negative (-) terminals, and just contact the frame of the flashlight. You'll want to solder on a wire to one of the large negative solder joints, about 1" (25mm) long. Strip the other end, make a hook, and tin that hook with solder for later.

To test for functionality as shown, connect your battery pack with the red/positive lead touching the center where the spring was and the negative to one of those edge joints - it looks really cool from the back when you turn off the lights!

Step 3: Wire in the Microswitch

Take a look at your microswitch. It should have three little terminals on the bottom, and a little spring lever arm at the top. Remove the spring at the top - I just used my flush cutters, some diagonal cutters should be fine. There should be some letters on the switch that say "NO", "NC" and "C (or COM)". These stand for Normally Open, Normally Closed, and Common. How these switches work is two different ways - in Normally Open, the switch is open all the time except for when pressed, which then turns on whatever it's connecting to power. In Normally Closed, the opposite is true - whenever the button is depressed, the switch turns off connection to power and the rest of the time the circuit is active. C or COM is for the Common terminal, which is generally where the power or load is connected. The button on the switch connects between C/COM and either NO or NC.

We need to use Normally Open (NO) and Common (C). If you wire it any other way it won't work, so be mindful of this! I connected the black (ground or negative) wire from the battery supply up through the hole in the bottom below the rectangular recess to the NO terminal. Pull it through the hole, make a hook with the exposed wire, and tin this connection. This is good practice when working in small enclosures like this, it will make it much easier to solder this connection, promise. I had to cut the wire from the battery pack back a bit, and used the extra length in the previous step to wire to the light for the connection to the negative terminal.

Step 4: Wire in the UV LED Assembly and Test

Now we will add the UV LED assembly. Solder the little short black wire from the negative (-) solder terminal on the light that you have already soldered onto the UV assembly to the "C" or "COM" terminal of the switch. If you made a tinned hook, you can put it through the little hole in the terminals and touch with your iron, then add a bit more solder. This will make the connection both mechanical and electrical, and also makes it easier to work on in this tight space. Be careful not to touch your iron to the sides of the plastic as we fill it with these two items, it gets crowded in there fast and PLA melts fast.

Once you've wired the negative to the switch, take the red positive (+) wire that was fished through the center hole of the enclosure and solder it to the center of the UV LED assembly where the spring was removed.

Now, it is time to test your circuit - don't skip this part! It is ALWAYS good practice to test before closing up. Put your batteries in your power pack, turn on the switch on your power pack if there is one, and depress the button on the top of the microswitch. If the lights turn on, you are good to proceed. If not, check your solder connections and wiring. I accidentally wired the light to the NC terminal instead of common, but caught the error in testing before putting everything in place semi-permanently. It happens to the best of us!

Step 5: Finish Assembly and Test Pressure Switch

Push the microswitch into the recessed area for it. Now for what might be the tricky part in replicating this - I designed the enclosure so that the microswitch is slightly above the ridge that goes around the perimeter of the inside of the enclosure. If yours is shorter, and hangs below, it will not activate when the ball is placed inside. Add hot glue, foam, or whatever is handy to build it up so that the little button is above the ridge and the acrylic disc will rest on the button.

Then push the LED assembly into its home at the bottom of the enclosure. If this fits snugly, sand the edges a bit or heat the enclosure in that area slightly with a heat gun/hair dryer and press in. It's good to test along the way to make sure you don't rip any of the wires out when you assemble.

Put the acrylic disc inside the enclosure. Gently press on the disc - if all goes right, the microswitch button will be depressed and the UV LEDS will light up. Glue or tape the enclosure to the battery pack to make things all compact and tidy.

Step 6: Go Play in the Dark!

You and your dog are ready to have some fun! It only takes 5-10 seconds to get a good glow on the ball, and it lasts about 10-15 minutes. Once the glow starts to fade, simply place it in the device for another few seconds and repeat. I hope you and your dog enjoy hours of fun!

The batteries should last quite a long time given the short duration of charging. If you find that your dog slobbers a lot, you may want to glue around the connections at the bottom inside, but the acrylic disc should keep out most dirt, debris and dog slobber. It's also handy to have used the glowing PLA - the charger also makes it glow for a short duration, making it easy to put the ball in the right place in the dark! If your battery pack has a switch on it like mine, turn it off and you can store the ball in the enclosure somewhere without running down the batteries when it depresses the microswitch. Just turn it back on when you are ready to play again, which is all the time for my dog.

Step 7: (Optional) Learning Time

So, when I was building this device I learned quite a bit about how these flashlights work and some power considerations that I will bore anyone with who has the interest to read this far. I initially tried using an old free USB rechargeable power supply that I got for free with a $5 subscription to Wired magazine years ago. When I hooked it up to the already-disassembled UV LED assembly, the whole thing blinked intermittently and did not work properly. This was not the case when I connected it to my benchtop power supply at 5V. (USB power is also 5V, if you didn't know that already).

So, I hooked up my multimeter to measure the current consumption, and the power consumption of the assembly at 5V was a whopping 4 amps! Mind you, this is on my hacked PSU ATX power supply that can provide like 30A @5V without breaking a sweat (or fuse). It was also pretty darn bright. The cheap promotional USB supply is only 1A, so the LEDs were drawing too much current. I hadn't expected it to be above 1A, but it is an assembly of 51 LEDS in series and each consumes 20-30 milliamps. It was tripping some internal protection in the USB power supply because it was over-current.

I know from other electronics hacking that LEDs have different power consumption based on their wavelength. UV LEDs are not very efficient and on the high end of that range. See this article for more information than you would ever want to know about UV LED efficiency improvements. And for a neat demo on how LEDs put out a different wavelength at different temperatures, check out this video (there are a few others on YouTube) where people dip them in liquid nitrogen - I've tried it and it's pretty cool!

When measuring the 6V DC current from the battery pack hooked up to the assembly, it was still 2A. I suspect that the batteries are self-limiting in terms of current output, and know that most 1.5V dry-cell batteries have a pretty decent surge capacity - each one is like 1amp for short durations. I'm not 100% sure why this array draws so much current, if someone wiser than myself wants to post in the comments that would be fantastic. If you just want to use a few individual UV LEDs for this project and a smaller rechargeable USB or LiPo battery pack, it wouldn't charge the ball up as fast but also wouldn't have the power consumption of the whole array.